1. Field of the Invention
The present invention relates to the use of a chemical compound as an etchant for the removal of the unmodified areas of an imagewise modified chalcogenide based glass while leaving the modified areas un-removed. Further the present invention is directed to a method for the preparation of a free standing 3-D nano-structure by using the mentioned compound as well as to the compound.
2. Description of the Related Art
Chalcogenide glasses are materials that have at least one element in its chemical composition that belongs to group VI of the period table, namely the elements sulfur, selenium and tellurium. These chalcogenide glasses are able to be used as photo-resistive material.
A resist material is known as a material that can have its chemical behavior modified via the exposure to an external stimulus. This stimulus could be induced by photons, electrons or other types of energy beams.
One method to generate a photoinduced change in chalcogenide based glasses is the method of three-dimensional direct laser writing (3D-DLW). The method of 3-D DLW takes advantage of the fact that the method utilizes laser wavelengths that are below the bandgap of the photoresist material, so that one-photon absorption is negligible. What this means is that without focusing the laser radiation that is utilized, the beam can essentially pass through the material without being absorbed. However, if highly intense laser radiation is used, such as those generated from a femtosecond laser, and the laser beam is tightly focused into a very small spot, in the order of 200 nm, then the probability of the laser energy being absorbed via a two-photon process would likely occur. However, the absorption only occurs at the focus of the laser spot where the laser intensity is the highest, all other areas where the laser intensity is not focused will not be polymerized. This ability for the laser energy to be selectively absorbed in a three-dimensional space allows for the in-situ, or direct, generation of a three-dimensional image inside the photoresist. The images that are generated using the 3-D DLW method provide very tall, or thick, structures, with minimum feature sizes below 200 nm, directly inside the photoresist. To etch out, or reveal this structure then requires a highly selective chemical etchant that allows for the total removal of the unexposed areas, while leaving the exposed areas behind.
A liquid etchant is generally defined as a chemical solution that is able to controllably remove a solid material. When such an etchant is used in the field of resists, it has the meaning that it possesses the ability to controllably remove areas that have not been modified using an external stimulus, (ie. photons, electron beams, etc), over that of those areas that have been modified using an external stimulus, or vice versa. In the instance described in the following, the removal of the unmodified areas over that of the modified areas serves to provide a negative image of the laser-beam or photo-mask, therefore this etchant is a negative tone etchant.
Liquid etchants for chalcogenide glasses have been formulated to take advantage of the fact that there is a chemical difference that arises in chalcogenide-based glasses after exposure to an external stimulus. There are two main types of liquid etchants that are known in the scientific and patent literature which etch chalcogenide-based glasses. These two types mainly differ by the solvent systems that they employ. The first type are water-based (aqueous), and the second are organic solvent based etchants.
In the aqueous based etchants, the active component in their composition is usually an inorganic base which generates a basic condition (pH>7) using anions such as hydroxide, sulfides, sulfites, disulfides, amines and cyanides. The basic environment creates oxidizing conditions which oxidize the components in the chalcogenide glass into water-soluble species, and thereby dissolving them in the aqueous phase. Additives such as surfactants are also added to increase the selectivity of the etchant composition. It has been shown that etchants containing these species are able to etch chalcogenide-glasses with some selectivity but they have only been shown to be confined to etching very thin films with structures having a height of no more than 500 nm. This is due to the fact that the oxidizing conditions are so strong, that all of the material whether modified or not, will be completely removed after a short time.
In the organic based etchants, the active components in its composition is usually a short chain amine, C<5, that is dissolved in an organic solvent. These compositions have also been shown to reveal thin 2-D structures usually no more than 500 nm thick. The organic amine again acts as an oxidant to oxidize the species in the chalcogenide-based glasses. Only very thin structures are made because the oxidizing conditions are again so strong that all of the material is removed, and hence the reaction is essentially kinetically controlled.
It has been shown that the photosensitive V-VI semiconducting chalcogenide glass, As2S3, is also compatible with the 3-D DLW process. But the etching chemistry in As2S3 is not developed well enough to take full advantage of the precision placement of 3-D features, and sub-diffraction limit resolution, that the 3-D DLW method routinely achieves in organic photopolymers (Wong et al. Adv. Mater. (2006), 18, pg 265-269).
The problem that was noticed earlier on when fabricating 3-D structures on the nano-scale was that the demands on the etchant's selectivity (γ), becomes much greater than for 2-D structures. The selectivity of an etchant (γ), is defined as the ratio between the rate of the unexposed areas (Ku) and the rate of the removal of the exposed areas (Ke). Therefore, when developing As2S3 as a negative-tone photoresist, the unexposed areas should be removed as quickly as possible, while the exposed areas should remain as long as possible, so as to afford a large γ. A large γ will allow thick structures with fine resolution to be produced.
The first demand on an etchant system that requires a large γ, stems from the fact that 3-D structures must be immersed for a longer period of time in the etchant to develop its structures. This is a result of the slower diffusion of liquid etchants through a more obstructed nano-porous structure. Another demand is due to the higher reaction rate on the areas that have already been developed by the etchant in 3-D structures. This is because the surface area of a porous 3-D structure is much larger than the surface area of a 2-D structure of the same volume. Therefore, the etch selectivity of the etchant that is employed to directly develop a 3-D structure, must be significantly higher in the 3-D case than the 2-D case. Currently available etchant studies found in the literature (A. Feigel et al. Applied Physics letters (2003), 83, pg 4480.) that deal with As2S3 as a photoresist, are only investigated to be applied to fabricate thin 2-D structures less than 500 nm in height. There, the etch selectivity requirements are much lower. Functional 3-D structures (such as 3-D PBG materials, or nano and micro-machines) often require structures with a height of 20 microns or more. Therefore, the demands placed on the etch selectivity of the etchant composition for direct 3-D fabrication of 3-D nano-structures are much greater.